Vehicle cruise control permits a vehicle driver to set a predetermined road speed regardless of gradient. Cruise control is generally enabled for highway driving only, typically at speeds greater than 30 kph.
Autonomous cruise control (also known as adaptive cruise control or ACC) relies on forward facing radar to permit a vehicle to follow another vehicle at a predetermined separation. The speed of the following vehicle is varied automatically with the speed of the leading vehicle, within certain pre-set speed and acceleration limits. Autonomous cruise control is typically used in highway driving where the frequency of interruption is small, and where speed variation is within a relatively narrow band.
Certain control strategies have been developed for autonomous cruise control at highway speeds. Thus a lead vehicle may disappear from view, for example owing to a bend in the road; in this circumstance the following vehicle will accelerate towards a pre-set upper speed limit until the leading vehicle again comes into view, whereupon the following vehicle reduces speed to maintain the desired separation distance. The rates of acceleration and of deceleration in cruise control mode are limited in order to ensure smooth response of the vehicle to changes in separation distance. Greater rates of deceleration may be provided in other automatic systems, such as collision avoidance systems, but these are not typically part of cruise control functionality.
It will be appreciated that control strategies suitable for autonomous cruise control in highway driving may not be suitable for urban use where vehicle speed may vary across the entire permitted range, where speed variation is frequent, and where the frequency of interruption is high.
Thus two vehicles. the following vehicle using autonomous cruise control, moving at a typical urban speed of 30 kph may approach a corner. The leading vehicle maintains speed but moves out of view of the following vehicle. In consequence the following vehicle accelerates to bring the leading vehicle back into view, but in doing so the following vehicle accelerates into the corner against driver expectation, which may be unnerving to the vehicle occupants.
It is against this background that the present invention has been conceived. Embodiments of the invention may provide a method or system of autonomous cruise control which is suitable for use in urban conditions. Other aims and advantages of the invention will become apparent from the following description, claims and drawings.